Question about critical section when enable MPU support

Kernel
1

I am porting the PMP part of RISCV with reference to MPU. I am currently confused about whether non-privileged tasks can enter the critical section.

On ARM Cortex-M3/4/7 ports, there is a macro configALLOW_UNPRIVILEGED_CRITICAL_SECTIONS to allow unprivileged tasks to create critical sections. In FreeRTOS-Kernel/portable/GCC/ARM_CM4_MPU/portmacro.h at e5987bbdb2041f70c9502f99a8c5643a30b9ff26 · FreeRTOS/FreeRTOS-Kernel · GitHub . vPortEnterCritical() and vPortExitCritical() are not placed in the PRIVILEGED_FUNCTION section. I’m wondering if unprivileged tasks can call taskENTER_CRITICAL() if configALLOW_UNPRIVILEGED_CRITICAL_SECTIONS is disabled?

Besides, for Cortex-V8M (CM23, CM33), can unprivileged tasks enter critical sections?

2

Unprivileged tasks are not permitted to enter critical sections because, if allowed, a misbehaving unprivileged task could enter a critical section and deliberately not exit, bringing the system to a halt. The configALLOW_UNPRIVILEGED_CRITICAL_SECTIONS configuration option in ARM-v7M ports exists solely for backward compatibility and its use is discouraged.

For more details on the security considerations, please refer to the FreeRTOS Kernel Threat Model: Kernel threat model - FreeRTOS™.

Once done, please upstream your port as it could benefit the broader FreeRTOS community.

3

Thanks for confirming—absolutely want to contribute if possible.

But there seems to be significant uncertainty here, because RISC-V’s PMP (Physical Memory Protection) does not govern memory access permissions in M-mode—unless the lock bit is set. This stands in sharp contrast to Cortex-M’s MPU (Memory Protection Unit), which cancontrol access for privileged modes.

Additionally, pvPortMalloc()does not support specifying alignment granularity. If configSUPPORT_DYNAMIC_ALLOCATIONis enabled, on platforms with a large PMP granularity (e.g., 256 Byte), RTOS objects allocated from the FreeRTOS heap fail to meet the required alignment.

It seems there is still a long road ahead.

4

I am not very familiar with PMP but does it mean that we need to run tasks in U-mode and only kernel runs in M-mode?

For this one, you can enforce that all the tasks must be created using xTaskCreateRestricted APIs and the application writer must supply the correctly aligned stack.